1. Field of the Invention
The present invention relates to a display device for a cathode ray tube or for a liquid crystal display device including an anti-reflection film formed on the display plane.
2. Description of the Related Art
In general, the display screen of a cathode ray tube or a liquid crystal display device comprises a smooth glass substrate forming the outer surface of the display screen. As a result, a primary reflection of the external light takes place on the outer surface so as to obscure the picture image displayed on the display screen.
A cathode ray tube is constructed as shown in, for example, FIG. 1. It is seen that a phosphor screen 2 positioned within a vacuum envelope is formed inside a faceplate 1 made of a smooth glass plate. Also disposed within the vacuum envelope are an electron gun 3, which emits an electron beam 4, and a deflection yoke 5 serving to magnetically deflect the electron beam 4 so as to scan the electron beam in the horizontal and vertical directions on the phosphor screen 2. A picture image is displayed on the phosphor screen as a result of the horizontal and vertical scanning of the electron beam noted above. What should be noted is that the outer surface of the faceplate is made of a smooth glass, with the result that a primary reflection of an external light taking place on the outer surface of the faceplate 1 overlaps with the picture image displayed on the phosphor screen. Naturally, the overlapping noted above obstructs a clear viewing of the picture image displayed on the phosphor screen.
A means for overcoming the above-noted difficulty is proposed in, for example, Published Unexamined Japanese Patent Application No. 61-29051. It is proposed that fine concavities are formed on the outer surface of the faceplate so as to scatter the external light. In this method, however, the entire screen is rendered whitish, with the result that the contrast is seemingly deteriorated. Also, resolution of the picture image is deteriorated.
It is also known to the art that an optical film of a single layer structure or a multilayer structure is formed on the outer surface of the faceplate so as to prevent the reflection of the external light. This technique certainly permits effectively preventing the primary reflection an external light so as to prevent deterioration in contrast and resolution.
Where the optical film noted above is of a single layer structure, the optical film is formed of a material having a refractive index smaller than that of the glass forming the ordinary faceplate. It should be noted that the condition for eliminating reflection in this case is represented by: n.sub.1.sup.2 =n.sub.0 .multidot.n.sub.2.sup.2, where n.sub.1 denotes the refractive index of the film, n.sub.0 denotes the refractive index of the air, and n.sub.2 is the refractive index of the substrate in the display section.
In the case of a cathode ray tube, the substrate in the display section is formed of glass, i.e., n.sub.2.sup.2 =1.52. On the other hand, the refractive index of the air is: n.sub.0 =1.00. It follows that the refractive index of the optical film is: n.sub.1 =1.23.
In general, the optical film of the single layer structure is formed by means of vacuum vapor deposition of magnesium fluoride (MgF.sub.2). However, magnesium fluoride itself has a refractive index of 1.38, which is greater than 1.23 given above. It follows that the optical film formed by means of the vacuum vapor deposition fails to produce a sufficient effect of preventing the external light reflection. It is also known to the art to use fine magnesium fluoride particles for forming an optical film of a single layer structure. In this case, the magnesium fluoride particles are bonded to each other by silicon dioxide obtained by hydrolyzing and dehydrating-condensing for example, a silicon alkoxide.
FIG. 2 schematically shows an optical film utilizing fine magnesium fluoride particles. As apparent from the drawing, the film is porous such that clearances 7 are formed between adjacent fine particles 6 of magnesium fluoride. It is possible to prepare a porous optical film having a refractive index smaller than 1.38, i.e., smaller than the refractive index of magnesium fluoride itself. For forming a porous magnesium fluoride film having a sufficiently high mechanical strength, however, it is necessary to use a large amount of a binder relative to the amount of magnesium fluoride, with the result that the refractive index of the porous film is increased so as to suppress the effect of preventing light reflection. What should also be considered is the surface structure of the optical film. Specifically, FIG. 3 schematically shows the surface structure noted above. As seen from the drawing, silicon dioxide and uncondensed Si--O--H structure are present on the surface of the fine magnesium fluoride particle 6. The optical film of this surface structure readily adsorbs water. The adsorbed water permeats into the clearance within the porous film so as to bring about a change in the refractive index. In other words, the refractive index of the porous film is changed with time in accordance with increase in the amount of water adsorbed by the film.
Where the optical film is of a two layer structure, the condition for eliminating an external light reflection can be represented by: n.sub.2 .multidot.n.sub.4.sup.2 =n.sub.0 .multidot.n.sub.3.sup.2, where n.sub.2 denotes the refractive index of the glass substrate in the display section, n3 denotes the refractive index of the lower of the film, n.sub.4 denotes the refractive index of the upper layer of the film, and n.sub.0 denotes the refractive index of the air. Where n.sub.2 =1.52 and n.sub.0 =1.00, the ratio n.sub.3 /n.sub.4 is 1.23. It follows that is possible to prepare to desired optical film of a multilayer structure by suitably selecting the materials of the lower and upper layers of the film such that the refractive index ratio n.sub.3 /n.sub.4 is 1.23. In practice, an optical film having a desired refractive index can be readily prepared by using a material having a relatively large refractive index for forming the lower layer.
An optical film of a two layer structure, which permits eliminating an external light reflection, is disclosed in, for example, Published Unexamined Japanese Patent Application No. 61-10043. It is taught that the lower layer is formed of an oxide-series compound of titanium or zirconium, e.g., a co-condensate between an alkoxide of titanium or zirconium and a silicon dioxide series compound such as a silicon alkoxide. On the other hand, the upper layer is formed of a fluorine-containing silicone compound which can be obtained from an alkoxy silane or chlorosilane having a polyfluoroalkyl group. In the optical film of this type, however, the lower layer has a high refractive index. If some portion of the lower layer is not covered with the upper layer because of, for example, an uneven coating of the upper layer, a surface reflection takes place in the optical film in the portion where the lower layer is not covered with the upper layer, leading to a high reflectance. Suppose the lower layer has a refractive index of, for example, 1.7, the lower layer exhibits a reflectance of 10%, which is more than twice the reflectance 4.3% of the substrate formed of the ordinary glass. It follows that only a slight defect of the laminate structure attracts attentions. Naturally, the optical film of a two layer structure is lower in yield than the film of a single layer structure. Also, the film of a two layer structure requires a complex manufacturing process, leading to a high manufacturing cost, compared with the film of a single layer structure.
As described above, the conventional optical film for preventing an external light reflection in a display device leaves room for further improvements in reflectance, mechanical strength, humidity resistance and menufacturing cost.